XAML, eXtensible Application Markup Language, is a markup language for declarative application programming (a way of representing .NET object graphs in XML). It's main use is for defining user interfaces in wpf (Windows Presentation Foundation), silverlight, windows-phone and windows-8 store apps; the last two utilize principles from Microsoft Design Language (previously known as Metro or Modern UI), though it can also be used for other applications, notably wf (Windows Workflow Foundation) workflows.